The present invention relates to equipment of the type used in the process control industry. More specifically, the invention relates to diagnostics for process control device in which the diagnostics is a function of a resistance.
Process control devices are used to monitor process variables and control industrial processes. For example, a process control transmitter might monitor temperature and transmit such information back to a control room. Furthermore, a process controller such as a valve controller is used to control the process.
As sensors, control elements or other components, are subjected to harsh environmental conditions, the accuracy of the system tends to degrade. It is possible to compensate for this degradation by periodically recalibrating the device. Typically, this requires an operator to enter the field and perform a calibration process on-site on the device. This is both inconvenient and time consuming for the operator. Further, it is difficult to determine the condition of a device, prior to its ultimate failure.
It is also necessary for the device or their components to be periodically replaced as they age. However, it is difficult to determine precisely when such replacement is necessary. Therefore, components are typically replaced well before their failure or, in some cases, they may fail unexpectedly requiring an unscheduled system shutdown.
A device in a process control system includes an electrical element which has a resistance. self-heating circuitry coupled to the element provides a self-heating signal related to the resistance of the electrical element. Diagnostic circuitry provides an output as a function of the self-heating signal output, for example, a residual life estimate of the element or a calibration output.